Ordered arrays of crystalline-Si (c-Si) microwires, fabricated by chemical-vapor-deposition, vapor-liquid-solid (CVD-VLS) growth mechanism, were pioneered nearly five years ago for sunlight-to-electrical power conversion. P-type Si microwire arrays, employing a thin n+-doped emitter layer to form a buried junction (p-n+-Si), have since realized sunlight-to-electrical power-conversion efficiencies >7% from solid-state photovoltaic (PV) devices, and >5% power-conversion efficiency toward H2 evolution from acidic aqueous electrolytes when functionalized with Pt electrocatalysts. In the absence of additional processing-intensive steps for light absorption enhancement, these devices demonstrated a short-circuit (maximum) current density (jsc)≈9 mA/cm2, open-circuit (maximum) photovoltage (Voc)≈0.53 V, and fill factor≈70%. The product of these three terms determines the power-conversion efficiency of the device. The Si microwire geometry uses ˜5% of the material required for conventional wafer-based photovoltaics (PVs) and absorbs ˜20% of above bandgap sunlight. Various designs to alter the path of light and increase absorption by the Si microwire arrays, and thus jsc and the efficiency, have been investigated with modest success.
Si microwire array photocathodes have been shown to generate photovoltages in excess of 500 mV in acidic aqueous environments, and provide a desirable geometry, relative to planar structures, for devices that effect the unassisted generation of fuels from sunlight. Microwire arrays benefit from orthogonalization of the directions of light absorption and minority-carrier collection, as well as from light-trapping effects, an increased surface area for catalyst loading per unit of geometric area, a small solution resistance as compared to planar designs, a reduced material usage through reusable substrates; and from the ability to embed the microwires into ion exchange membranes that exhibit little permeability to H2 and O2, thereby producing flexible devices that persistently separate the products of the water-splitting reaction.
However, the voltage generated from single-junction Si microwire arrays is much lower than the 1.23 V required for solar-driven water splitting.